Line reflection reduction with energy-recovery driver

ABSTRACT

A system and method for reducing reflections in a transmission line and for recovering energy from the load in the transmission during the process. At least three drive signal levels are utilized. The transition from the second level to the third level during a rising transition and the transition from the second level to the first level during a falling transition is timed to coincide with the arrival of the reflected signal from the immediately-preceding transition. A capacitor is advantageously used as the source for the intermediate drive signal levels and advantageously facilitates energy recovery and conservation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 11/196,892,filed Aug. 4, 2005, now U.S. Pat. No. 7,176,712, entitled “LineReflection with Energy-Recovery Driver,” which is a continuation ofapplication Ser. No. 10/776,927, filed Feb. 10, 2004, now U.S. Pat. No.6,946,868, entitled “Line Reflection Reduction with Energy-RecoveryDriver,” which is a continuation of application Ser. No. 10/270,851,filed Oct. 14, 2002, now U.S. Pat. No. 6,696,853, entitled “LineReflection Reduction with Energy-Recovery Driver,” which is acontinuation of application Ser. No. 09/532,798, filed Mar. 21, 2000,now U.S. Pat. No. 6,486,697, entitled “Line Reflection Reduction withEnergy-Recovery Driver,”. The contents of all of these applications areincorporated herein by reference.

Application Ser. No. 09/532,798 claimed the benefit of U.S. ProvisionalApplication No. 60/125,580, filed Mar. 22, 1999, entitled “LineReflection Cancellation with Energy-Recovery Driver,” the contents ofwhich are also incorporated herein by reference.

GOVERNMENT LICENSE RIGHTS

The invention was made with government support under DAAL01-95-K-3528sponsored by DARPA and MDA904-93-C-L042 sponsored by DOD. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to line drivers and, more particularly, totechniques for reducing ringing and power losses in line drivingsystems.

2. Description of Related Art

Line reflection decreases the noise margin in high-speed digitalcircuits, especially line reflections induced by signal buffers drivingoff-chip loads.

The signal buffer acts as a line driver. After the driver causes asignal transition, the transition travels from the source (near) end ofthe transmission line to the load (far) end. Upon reaching the end, thesignal transition is usually reflected at the far end and travels backtoward the source. The reflected signal is then usually again reflectedupon reaching the source back towards the load. This process continuesuntil the cumulated losses cause the reflection to die out. Theresulting voltage waveform seen at either end of the transmission lineis typically an exponentially-damped oscillation, often referred to as“ringing.”

Ringing often creates problems. It often causes the voltage on thetransmission line to exceed allowable or safe levels. Therefore,circuitry connected to the line must be designed to accommodate highervoltage levels than are actually needed. The ringing can also beerroneously interpreted as a change in the state of the data on theline.

In the past, efforts have been made to match the impedance of the driverto the transmission line and/or the impedance of the transmission lineto the load. If the impedance at one end or the other is perfectlymatched, there would normally be no reflection.

A simple approach used to match the impedance between the transmissionline and the load is to connect a resistance at the end of thetransmission line to ground. This approach, however, causes additionalpower to be dissipated in the resistance that is added. This isundesirable in low-power applications, such as in a VLSI pin driver usedfor fast chip-to-chip communication.

A simple approach for matching the impedance between the driver and thetransmission line is to insert a resistance in series between the outputof the driver and the input of the transmission line. Again, however,the addition of such a resistance increases power dissipation.

Another problem with line driving systems is the dissipation of powerthat occurs during transitions of the signal. This is particularly truewhen the load includes a substantial capacitive reactance, such as in aVLSI pin driver used for fast chip-to-chip communication.

In short, there is a need for a driver that drives a transmission lineconnected to a load which substantially reduces ringing without wastingpower and which, preferably, reduces the energy that is dissipatedduring operation.

SUMMARY OF INVENTION

One object of the invention is to obviate these as well as otherproblems in the prior art.

Another object of the invention is to reduce ringing intransmission-line driving systems.

A still further object of the invention is to reduce ringing intransmission-line driving systems without increasing power dissipation.

A still further object of the invention is to reduce the maximumvoltage-level specification of circuitry that is connected to atransmission-line driving system.

A still further object of the invention is to reduce data errors causedby transient signals in transmission-line driving systems.

A still further object of the invention is to conserve energy consumedby a transmission line and the load it drives.

These as well as still further objects of the invention are achieved byan apparatus and method that transition the input signal to atransmission line in a plurality of steps and that cause at least one ofthose steps to coincide with the arrival of a reflected signal back atthe input of the transmission line.

In one embodiment of the invention, a signal generation system generatesat least a first drive signal, second drive and third drive signal, thesecond drive signal having a level greater than the first drive signal,and the third drive signal having a level greater than the second drivesignal. A controller is in communication with the signal generationsystem to cause the signal generation system to deliver the first drivesignal, then second drive signal, and then third drive signal to theinput of the transmission line. The third drive signal begins to bedelivered to the input of the transmission line approximately when areflection of the second drive signal from the output of thetransmission line first arrives at the input to the transmission line.

In a still further embodiment of the invention, the controller alsocauses the signal generation system to deliver the third drive signal,then second drive signal, and then drive first drive signal to the inputof the transmission line. During this phase, the first drive signalbegins to be delivered to the input of the transmission line atapproximately when a reflection of the second drive signal from theoutput of the transmission line first arrives at the input to thetransmission line. In this embodiment, the signal generation systempreferably includes a source of the second drive signal that includes anenergy storage device, such as a capacitor. In a preferred embodiment,the capacitor receives all of its charge solely from the transmissionline.

In a still further embodiment of the invention, the level of the seconddrive signal is approximately midway between the level of the firstdrive signal and the third drive signal.

In a still further embodiment of the invention, the level of the seconddrive signal is approximately equal to the reflected level of the firstdrive signal and the reflected level of the third drive signal.

In a still further embodiment of the invention, the signal generationsystem also generates a plurality of drive signals, in addition to thefirst drive signal, second drive signal and third drive signal. In apreferred embodiment, the source for each of the plurality of additionaldrive signals includes an energy storage device, such as a capacitor.Preferably, each capacitor receives all of its charge solely from thetransmission line.

In a still further embodiment of the invention, the signal generationsystem includes a supply for generating each of the drive signals and aswitching system that selectively connects each of the drive signals tothe input of the transmission line. In this embodiment, the controllercontrols the switching system.

The invention also includes a process for driving a transmission lineconnected to a load, the transmission line having an input and anoutput.

In one embodiment of the process, a first drive signal, second drivesignal and third drive signal is generated. The second drive signal hasa level greater than the first drive signal; and the third drive signalhas a level greater than the second drive signal. The first drivesignal, second drive signal and then third drive signal is delivered tothe input of the transmission line. The delivery of the third drivesignal begins approximately when the reflection of the second drivesignal from the output of the transmission line first arrives at theinput to the transmission line.

In another embodiment of the process, the third drive signal, seconddrive signal and then first drive signal is also delivered to the inputof the transmission line. In this embodiment, the first drive signalbegins to be delivered to the input of the transmission line atapproximately when a reflection of the second drive signal from theoutput of the transmission line first arrives at the input of thetransmission line.

In a still further embodiment of the process, a source is used toprovide the second drive signal and includes an energy storage device,such as a capacitor. Preferably, the capacitor receives all of itscharge solely from the transmission line.

In a still further embodiment of the process, the level of the seconddrive signal is approximately midway between the level of the firstdrive signal and the third drive signal.

In a still further embodiment of the process, the level of the seconddrive signal is somewhat above the midway level on the rising transitionand somewhat below the midway level on the falling transition.

In a still further embodiment of the process, a plurality of drivesignals are generated, in addition to the first drive signal, seconddrive signal and third drive signal. Preferably, a source is used togenerate each of the additional drive signals, each source including anenergy storage device, such as a capacitor. Preferably, each capacitorreceives all of its charge solely from the transmission line.

In a still further embodiment of the process, a supply generates each ofthe drive signals and a switching system selectively connects each ofthe drive signals to the input of the transmission line. In thisembodiment, a controller controls the switching system.

In a still further embodiment of the invention, a driver drives atransmission line having an input connected to the driver and an outputconnected to a capacitive load. The driver includes a high-potentialvoltage source; a first electronically-controlled switch connectedbetween the high-potential voltage source and the input of thetransmission line; a low-potential voltage source; a secondelectronically-controlled switch connected between the low-potentialvoltage source and the input of the transmission line; an energy storagedevice, such as a capacitor; a third electronically-controlled switchconnected between the capacitor and the input of the transmission line;and a controller connected to the first, second and thirdelectronically-controlled switches. The controller causes the thirdelectronically-controlled switch to close approximately when a signalinjected into the transmission line by the secondelectronically-controlled switch returns back to input of thetransmission line. The controller also causes the firstelectronically-controlled switch to close approximately when a signalinjected into the transmission line by the secondelectronically-controlled switch returns back to the input of thetransmission line.

These as well as still further objects, features and benefits of theinvention will now become clear from a review of the following detaileddescription of the preferred embodiments, read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a prior art line driver system.

FIG. 2 illustrates the ringing that typically occurs in the prior artline driver system shown in FIG. 1.

FIG. 3 illustrates a line driver system made in accordance with oneembodiment of the invention.

FIG. 4 illustrates the reduction in the ringing that typically occurswith the embodiment of the invention shown in FIG. 3.

FIGS. 5( a) and (b) illustrate two other input wave shapes that areuseful in other embodiments of the invention.

FIG. 6 illustrates a line driver system made in accordance with anotherembodiment of the invention that additionally provides energy recovery.

FIG. 7 illustrates a line driver system made in accordance with anotherembodiment of the invention that drives a plurality of transmissionlines of varying length.

FIG. 8 illustrates another embodiment of the signal generation system ofthe invention.

FIG. 9 is an alternate embodiment of the line driver system shown inFIG. 6 containing a replenishing system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a prior art line driver system.

As shown in FIG. 1, a transmission line 1 delivers a voltage V_(L) to aload 3.

The transmission line 1 is driven by a driver. The driver includes asignal generation system, including a switch 5, a first drive signal 7(which is shown in this example as being ground) and a second drivesignal 9 (which is shown as being a source of voltage V). The signalgeneration system is connected to a controller 11 which electronicallycontrols the switch 5, thus causing V_(IN) at an input 13 of thetransmission line 1 to switch between the first drive signal 7 (ground)and the second drive signal 9 (V). In a typical configuration, theswitch 5 is an electronic switch, such as a FET, MOSFET, SCR, triac orBJT.

The goal of the prior art line driver system shown in FIG. 1, of course,is to cause V_(L) at an output 15 of the transmission line that isacross the load 3 to switch between the levels of the first drive signal(ground) and the second drive signal (V) in synchronism with the controlsignal generated by the controller 11. In a typical system, thecontroller 11 receives data as a serial stream of logical “ones” and“zeros.” The net result is that the data is delivered to the load 3.

In practice, however, the signal that is generated by a transition(rising or falling) in V_(IN) at the input 13 to the transmission line 1is reflected at the output 15 of the transmission line 1 back to theinput 13. This occurs when, for instance, the impedance of the load 3does not match the impedance of the transmission line 1.

When the reflected signal arrives back at the input 13 to thetransmission line 1, it is usually again reflected back to the output 15of the transmission line 1. This typically occurs because the impedanceof the signal generation system is also not the same as the impedance ofthe transmission line 1.

This now twice-reflected signal is again reflected when it reaches theoutput 15 of the transmission line 1 back to the input 13. This processrepeats until the losses in the system reduce the magnitude of areflection to zero. Typically, the magnitude of the reflected signaldecays exponentially. This phenomena is often referred to as “ringing”because of its oscillatory nature.

FIG. 2 illustrates the ringing that typically occurs in the prior artline driver system shown in FIG. 1. The top of FIG. 2 illustrates atypical profile 21 for V_(IN). As shown in this profile, V_(IN)typically transition from a first voltage level (0 in this example) to asecond voltage level (V in this example) and then later transitions backto the first voltage level.

A profile 23 is shown of the voltage V_(L) that is delivered across theload. As shown by FIG. 2, however, V_(L) does not always match V_(IN).Not only is it slightly delayed in time (due to the time it takes thesignal to travel through the transmission line 1), but there isnoticeable “ringing” following each transition.

Of course, it is to be understood that the magnitude of the ringing, aswell as the number of cycles in the ringing, will vary widely, dependingupon the parameters of the system.

As indicated above, this ringing can be quite problematic. It exposescircuitry connected to both the input 13 and the output 15 of thetransmission line to higher-than-normal voltage excursions, requiringthe voltage ratings on these components to be increased beyond whatwould otherwise be needed. The ringing itself can also be misinterpretedby the circuitry as constituting a change in the data signal, creatingthe possibility of a data error.

FIG. 3 illustrates a line driver system made in accordance with oneembodiment of the invention.

As in FIG. 1, FIG. 3 includes a transmission line 1 having an input 13and an output 15 connected a load 3. Unlike FIG. 1, however, the systemshown in FIG. 3 utilizes three drive signals, a first drive signal 17(shown as ground), a second drive signal 19 (shown as V/2) and a thirddrive signal 21 (shown as V). The input 13 to the transmission line 1 isconnected to a switching system 23 that causes the input 13 to thetransmission line 1 to switch between one of the drive signals. Theswitching system 23, in turn, operates under the control of a controller25.

FIG. 4 illustrates the reduction in the ringing that occurs with theembodiment of the invention shown in FIG. 3. As shown in FIG. 4, thecontroller 25 causes the switching system 23 to switch the input 13 ofthe transmission line 1 from the first drive signal 17 (shown asground), to the second drive signal 19 (shown as V/2) and then to thethird drive signal 21 (shown as V). Thereafter, the controller 25 causesthe input 13 to the transmission line 1 to be connected back again tothe second drive signal 19 (shown as V/2) and then the first drivesignal 17 (shown as ground). This is illustrated in a profile 31.

The corresponding output voltage V_(L) is illustrated in a profile 33 inFIG. 4.

As shown in FIG. 4, the load V_(L) begins to transition from the firstvoltage level (ground) to the second voltage level (V/2) at time t_(d)after the input voltage V_(IN) makes this transition. The time t_(d)represents the time it takes a signal to travel from the input 13 of thetransmission line 1 to the output 15 of the transmission line 1.

As soon as the transition is received at the output 15 of thetransmission line 1, it is reflected back to the input 13 because of theimpedance mismatch. However, the signal from the input 13 is stilltraveling to the output 15. The reflected signal is therefore added tothe incoming signal, causing V_(L) at the output 15 of the transmissionline 1 to rise all the way up to the level of the third input signal(V), as also shown in FIG. 4.

The controller 25 is configured to cause the switching system 23 toswitch the input 13 to the third drive signal 21 (V) at approximatelythe moment the reflected signal first arrives back at the input 13 tothe transmission line 1. The reflected signal is thus met with a newincoming signal, which is approximately equal in magnitude, thussubstantially reducing any further reflections, even though there mightbe an impedance mismatch between the impedance of the transmission lineand the impedance of the drive system. Mathematically, the step from thesecond drive signal level to the third drive signal level is timed to beapproximately twice the signal delay time of the transmission line 1,2t_(d), as also illustrated in FIG. 4.

The net result is that the ringing is reduced, as also illustrated inFIG. 4. Accordingly, the output voltage V_(L) across the load 3transitions smoothly from the level of the first drive signal (ground)to the level of the third drive signal (V) without noticeable ringing.

The reverse process is preferably followed during the downwardtransition of V_(IN) from the level of the third drive signal (V) to thelevel of the first drive signal (ground), as also shown in FIG. 4. Asshown in FIG. 4, this downward transition is also made in two steps. Thefirst step is a transition from the level of the third drive signal (V)to the level of the second drive signal (V/2). Following a delay ofapproximately 2t_(d) (the time needed for the transition to the level ofthe second drive signal to be reflected from the end 15 of thetransmission line 1 back to the beginning 13 of the transmission line1), the second transition to the level of the first drive signal(ground) is made at the output 15. As shown in FIG. 4, this similarlycauses the output voltage V_(L) across the load 3 to transition smoothlyfrom the level of the third drive signal (V) to the level of the firstdrive signal (ground) without appreciable ringing.

The voltages of the various drive signals can vary widely. Although thelevel of the first drive signal 17 is shown as being ground, it is to beunderstood that the level of the first drive signal 17 could, in fact,be a negative or a positive voltage with respect to ground. The level ofthe second drive signal 19 is between the level of the first drivesignal 17 and the level of the third drive signal 21, as in the examplediscussed above.

In one embodiment, the level of the second drive signal 19 is midwaybetween the level of the first drive signal 17 and the level of thethird drive signal 21.

In another embodiment, the magnitude of the reflected signal thatarrives back at the input 13 may be something less than twice themagnitude of the signal that is sent to the output 15. This can happen,for example, when the impedance of the load 3 is somewhat matched to theimpedance of the transmission line 1. In this case, the level of thesecond drive signal is somewhat above the midway level on the risingtransition, so that the level of the third drive signal matches thereflected level of the second drive signal; and the level of the seconddrive signal is somewhat below the midway level on the fallingtransition, so that the level of the first drive signal matches thereflected level of the second drive signal.

In a still further embodiment, the level of the second drive signal 19is midway between the level of the first drive signal 17 and the levelof the third drive signal 21, even when the impedance of the load 3 issomewhat matched to the impedance of the transmission line 1. Althoughthis will not reduce the ringing as much, it is often easier to generatesuch a midway level signal.

The timing of each rising transition from the level of the first drivesignal 17 to the level of the second drive signal 19 and the timing ofthe falling transition from the level of the third drive signal 21 tothe level of the second drive signal 19 are usually governed by thetiming of the data stream that is to be sent to the load 3 over thetransmission line 1.

On the other hand, the timing of the rising transition from the level ofthe second drive signal 19 to the level of the third drive signal 21 andthe timing of the falling transition from the level of the second drivesignal 12 to the level of the first drive signal 17 are governed by thelength of the transmission line 1 and, more particularly, by the time ittakes a signal to travel round-trip through the transmission line 1.This amount is noted on FIG. 4 as 2t_(d).

The controller 25 is configured to cause the switching system 23 tosequentially switch from the level of the first drive signal 17 to thelevel of the second drive signal 19 and then to level of the third drivesignal 21, and to then sequentially switch from the level of the thirddrive signal 21 to the level of the second drive signal 19 and then tothe level of the first drive signal 17 in accordance with these timingrequirements. As indicated above, the timing of the switch between thelevel of the first drive signal 17 and the level of the second drivesignal 19 on the rising edge and the switch between the level of thethird drive signal 21 and the level of the second drive signal 19 on thefalling edge are externally governed by the timing of the data streamthat is to be delivered. On the other hand, the timing of the risingtransition from the level of the second drive signal 19 to the level ofthe third drive signal 21 and the falling transition from the level ofthe second drive signal 19 to the level of the first drive signal 17 aregoverned by internal considerations, namely the time it takes theearlier drive signal to make a round trip through the transmission line1.

In one embodiment, this internal timing is established by empiricalcalculation or by experimental testing of an actual transmission line.Once determined, the controller 25 is programmed with this delayinformation, thus operating in an “open loop” mode.

In another embodiment, a sensing apparatus 24 is connected to the input13 of the transmission line 1 to detect when a reflection of atransition arrives back at the input 13 to the transmission line 1. Upondetection, the sensing apparatus 24 communicates with the controller 25to initiate the next transition that is needed. In this “closed loop”configuration, the second step is initiated based on the detection ofthe return of an actual reflection, not based on a prior calculation ormeasurement.

Although having thus-far described the input signal V_(IN) as beingcomposed of three drive signals each at different levels, it is to beunderstood that the input signal V_(IN) could be composed of a greaternumber of drive signals, such as 5 or 7, as shown in FIGS. 5( a) and(b), respectively. In this instance, of course, an equivalent number ofsupplies would be needed, one for each input level. Similarly, theswitching system, such as the switching system 23 shown in FIG. 3, wouldneed to be configured to sequentially switch between each drive signal.Correspondingly, the controller, such as the controller 25 shown in FIG.3, would need to be configured to cause the switching system tosequentially switch between the different drive signals. The timing ofthe first rising step and the first falling step would, again, beexternally governed by the timing of the data signal that is to bedelivered to the load 3. Each of the subsequent even steps (e.g., thesecond or fourth step) would be timed to begin the moment the reflectionof the transition from the immediately preceding step arrives back atthe beginning 13 of the transmission line 1. The timing of the remainingodd-numbered steps (e.g., steps 3 or 5) could vary. Preferably, though,the timing of the remaining odd-numbered steps would be short tomaximize the speed of data transfer. Indeed, each remaining odd-numberedstep could be made at the same time as its preceding even numbered step.In this embodiment, each pair of even and odd numbered steps would bemerged into a single step, thus reducing the total number of steps andsupplies.

One continuing problem when driving transmission lines with data signalsis energy losses that occur during signal transitions. These lossesbecome particularly great when the transmission line 1 has a largeresistance. These energy losses can be particularly problematic inlow-power applications, such as in a VLSI pin driver used for fastchip-to-chip communication.

FIG. 6 illustrates a line driver system made in accordance with anotherembodiment of the invention that, in addition to ring reduction,minimizes these energy losses. The system is the same as the systemshown in FIG. 3, except that the second drive signal 19 is generated bya capacitor 41, instead of a source, such as the V/2 shown in FIG. 3.This modification adds an energy-recovery function to the invention.

When first energized, the switch from the first drive signal 17 (groundin this example) to the second drive signal 41 will have no effect.V_(IN) will remain at the level of the first drive signal 17 (ground).The succeeding switch to the third drive signal 21 (V in this example)will then cause V_(IN) to jump to the third drive signal (V), much as inthe prior art system shown in FIG. 1. Unless the impedance of the loadis matched to the impedance of the transmission line 1, the outputvoltage V_(L) will suffer from ringing following this first transition,again much like in the system shown in FIG. 1.

During the first falling transition, the switching system 23 switchesfrom the third drive signal 21 (V) to the second drive signal 41.Although the level of the second drive signal 41 starts out at the levelof the first drive signal (ground in this example), the current thatflows into the capacitor 41 from the load 3 charges it to a levelbetween the first drive signal 17 (ground) and the third drive signal 21(V).

During the second rising transition, the switch from the first drivesignal 17 to the second drive signal 41 causes a small transition inV_(IN). The amount of ringing caused by the second rising transition isthus reduce somewhat.

During the second falling transition, the capacitor 41 will start out ata level between the third drive signal 21 and the first drive signal 17.This will reduce the ringing that is caused by the second fallingtransition somewhat. During the third rising transition, the initialvoltage across the capacitor 41 will be even higher, thus furtherreducing the ringing that is caused by the third rising transition.

After several rising and falling transitions, the voltage level on thecapacitor 41 stabilizes to approximately V/2, the level that in mostembodiments maximizes the reduction of ringing during both rising andfalling transitions. Thereafter, the system in FIG. 6 functions toreduce ringing, just like the system in FIG. 3.

Unlike the system in FIG. 3, however, the system in FIG. 6 isaccomplishing another important function—energy conservation. In bothFIG. 3 and FIG. 6, energy is delivered into the transmission line 1 andthe capacitive load 3 during each rising transition and is then removedfrom the capacitive load 3 and the transmission line 1 during eachfalling transition. In FIG. 3, however, most of the removed energy isdissipated as heat in the intrinsic resistances of the switching system23 and the internal impedances of the drive signal supplies. In FIG. 3,on the other hand, a significant portion of that returning energy isstored in the capacitor 41. Instead of being dissipated or otherwisewasted, this stored energy is then reused during the next risingtransition. The system in FIG. 6 thus conserves energy that wouldotherwise have been wasted, while simultaneously reducing ringing, allwithout adding any components to the system that dissipate additionalenergy.

Although having described this energy-recovery embodiment of theinvention as utilizing a capacitor, i.e., capacitor 41, other forms ofenergy-storage devices could be used instead, such as an inductor, acombination of an inductor or capacitor, or others combinations ofcomponents.

FIG. 7 illustrates a line driver system made in accordance with anotherembodiment of the invention that drives a plurality of transmissionlines of varying length. As shown in FIG. 7, a load 51 is driven at anoutput 53 of a transmission line 55 having an input 57; a load 61 isdriven at an output 63 of a transmission line 65 having an input 67; anda load 71 is driven at an output 73 of a transmission line 75 having aninput 77.

As also shown in FIG. 7, the lengths of the transmission lines 55, 65and 75 are different, and their inputs 57, 67 and 77 are all connectedto the output of a switching system 81. The inputs to the switchingsystem 81, such as inputs 83, 85 and 87, are each connected to a drivesignal. Although not shown, it is to be understood that the switchingsystem 81 may have additional inputs connected to other drive signals. Acontroller 83 controls switching of the switching system 81 to cause itto sequentially switch between the various drive signals. As with thesystems described in FIGS. 3 and 6, the first rising step and the firstfalling step are externally governed by the timing of the data signal tobe delivered. The remaining even steps are timed to coincide with thearrival of reflections, such that each reflection is met by anequivalent input signal, thus reducing any further reflection that mightotherwise be caused. As each new step introduced by the driver willtravel down all the transmission lines (with the exception of the linewhose incoming transition the new step served to cancel), the number ofreflected transmissions requiring new steps for canceling will growquickly unless the lengths of the transmission lines are related tosmall integers. As each reflected transition will in practice have beendamped somewhat compared to the outgoing transition, there will be apoint of diminishing returns where the power lost through ringing willbe smaller than the power spent in the line driver on generatingadditional steps.

FIG. 8 illustrates another embodiment of the signal generation system ofthe invention. Thus far, the switching systems, such as the switchingsystem 23 shown in FIGS. 2 and 6 and the switching system 81 shown inFIG. 7, have been illustrated as a single pole, multiple throw switch.Although such a choice simplifies the discussion that has thus far beenpresented, single pole, multiple throw electronic switches are notcommon. A more practical approach for implementing these switchingsystem is the approach shown in FIG. 8. As shown in FIG. 8, each drivesignal 91, 93 and 95 is connected to the input 97 of a transmission line(not shown) through its own electronic switch, such as electronicswitches 99, 101 and 103, respectively. Instead of simply controlling asingle switch, a controller 105 is configured to independently controleach of the electronic switches 99, 101 and 103, such that when oneelectronic switch opens, the next in sequence closes. Thus, the use of aplurality of single pole, single throw switches as the switching systemcan readily be adapted to any number of drive signals, such as the fivelevels shown in FIG. 5( a) or the seven levels shown in FIG. 5( b).

Again, each of these switches are preferably electronic, such as aMOSFET, SCR, triac or BJT. Other configurations known in the art for theswitching system, of course, are also contemplated.

FIG. 9 is an alternate embodiment of the line driver system shown inFIG. 6 with a replenishment subsystem. All of the components in FIG. 9are the same as those in FIG. 6 with the exception of a switch 117 andan additional connection 119 between the switch 117 and the controller23.

In operation, the voltage across the capacitor 41 may slowly declinefrom the V/2 value because of losses in the transmission line 1. Byperiodically activating the switch 117 from a control signal from thecontroller 23 over the control line 119, the V/2 value across thecapacitor 41 can be replenished. Preferably, this refreshing would bedone at a moment in time when the capacitor 41 is not connected to theline 1 through the switch 23. Of course, it could also be refreshed whenit is connected.

Although having thus far described certain embodiments of the invention,it is to be understood that the invention embraces many otherembodiments and configurations and has many other benefits. Theinvention is limited solely by the claims that now follow.

1. A method of reducing energy loss due to ringing on a transmission line that has one end connected to a driver having an energy storage system and another end connected to a load, the driver and the load having impedances different than the impedance of the transmission line, the method comprising driving the transmission line with a plurality of drive signals that are timed so as to cause energy in the ringing to be recovered by the energy storage system whereby the timing is controlled by a controller driving at least one electronic switch.
 2. The method of reducing energy loss due to ringing in a transmission line of claim 1 wherein the plurality of drive signals are dined so as to cause recovered energy in the energy storage system to be recycled back into the transmission line.
 3. The method of reducing energy loss due to ringing in a transmission line of claim 2 wherein the plurality of drive signals are timed so as to cause the recovery of energy by the energy storage system and the recycling of energy back into the transmission line to cyclically repeat in substantial synchronism with the ringing.
 4. The method of reducing energy loss due to ringing in a transmission line of claim 3 wherein the plurality of drive signals are dined so as to cause the magnitude of the ringing on the transmission line to be reduced.
 5. A driver for reducing energy loss due to ringing on a transmission line that has one end connected to the driver and another end connect to a load, the driver and the load having impedances different than the impedances of the transmission line, comprising: an energy storage system configured to store energy; and a signal generation system configured to deliver a plurality of drive signals to the transmission line that are timed to cause energy in the ringing to be recovered by the energy storage system whereby the timing is controlled by a controller driving at least one electronic switch.
 6. The driver of claim 5 wherein the plurality of drive signals are timed so as to cause recovered energy in the energy storage system to be recycled back into the transmission line.
 7. The driver of claim 6 wherein the plurality of drive signals are timed so as to cause the recovery of energy by the energy storage system and the recycling of energy back into the transmission line to cyclically repeat in substantial synchronism with the ringing.
 8. The driver of claim 7 wherein the plurality of drive signals are timed so as to cause the magnitude of the ringing on the transmission line to be reduced.
 9. The driver of claim 5 wherein the signal generation system is configured to generate signals having at least three different levels. 